Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry, reflectometry, and ellipsometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures.
In many semiconductor fabrication applications, a relatively thick, highly absorbing layer is deposited directly on a substrate (e.g., silicon substrate) or on top of a set of production film stacks fabricated on a substrate. In one example, a carbon layer having a thickness of one micrometer or more is deposited on a silicon substrate or on set of production film stacks deposited on the silicon substrate.
Optical measurements of the thickness of a relatively thick, highly absorbing layer are difficult due to the amount of light loss that occurs as the illumination light propagates through the layer thickness to the bottom of the layer and as the reflected light propagates back through the layer thickness to the top of the layer. As a result many optical techniques suffer from low signal-to-noise ratios (SNRs), as only a small fraction of the illumination light is able to reach the bottom of the thick, absorbing film, and reflect upwards to the detector. Thus, many available high-throughput metrology techniques are unable to reliably perform film thickness measurements of thick, absorbing film layers.
For example, a relatively thick carbon layer absorbs practically all measurement light in the ultraviolet and visible spectra. In response, attempts have been made to perform optical thickness measurements of a relatively thick carbon layer using infrared (IR) illumination to increase measurement sensitivity. IR illumination is employed because carbon is less absorbing in the IR spectrum compared to UV and visible spectra. Unfortunately, the reflectance from the silicon substrate in the IR spectrum is very low. This limits measurement sensitivity. Although, typical product stacks have higher reflectance in the IR spectrum, the measurement is complicated by the product stack and it has proven difficult to extract a measurement signal indicative of carbon layer thickness from signals arising from the product stack.
In summary, semiconductor fabrication applications involving thick, highly absorbing film layers impose difficult requirements on optical metrology systems. Optical metrology systems must meet high precision and accuracy requirements for the thickness measurement of thick, highly absorbing films at high throughput. Improved film architectures and metrology systems and methods are desired to overcome these limitations.